Those irrigation practices which are advantageous to mosquito control are also beneficial to the production of crops. At least 3 days in water are required for mosquitoes to reach the adult stage. With the exception of rice, water which is left standing for more than 24 hours after irrigation is of no benefit to the field in which it remains and usually becomes detrimental to the crop. In most cases, less than 1/2 inch of standing water in a field is removed by means of evaporation, this does not remove the water rapidly enough to promote mosquito control. The extent to which water infiltrates soil varies according to its texture and condition. Soils that are either fine textured, compacted, or excessively tilled, particularly when they are wet, may become almost impervious to water infiltration after a few hours. In this type of situation, the field must be graded with a slope to promote surface drainage for the excess water. Livestock should be kept out of wet fields to reduce compaction.
The proper grading of a field will help promote mosquito control as well as higher crop yield at reduced cost for both water and irrigation labor.
Design Criteria For Irrigated Pastures
All fields subject to irrigation should be leveled according to a designed grade with a minimum of soil movement (cutting and filling) and with a minimum down slope fall of 0.2 percent.
After leveling, the "strip check" or border method of irrigation should be established and utilized on the fields planted to pasture and field crops. The widths of borders should be no greater than 50 feet. Border levees should be constructed in such a way that no borrow pits will be left on their sides to collect and hold water.
The length of runs should be no greater than 1,320 feet. When establishing new head ditches, proper turnouts should be installed along with an outlet for draining the ditch. Tailwater drains should be constructed at the ends of all irrigated fields.
In all instances, water should be applied in quantities and at frequencies that will meet the needs of the crops grown on the particular soil types and that will provide for proper water penetration without erosion. To reduce mosquito production, water should not be allowed to stand in ditches or in fields longer than three (3) days.
The preceding mosquito prevention criteria are intended only to offer guidance when considering the development of design options during the planning process for projects. Be advised that these practices have been found to be effective, however, once the project has been completed it is essential that conscientious maintenance and management practices be followed to help ensure the successful prevention of mosquito production.
Be further advised that under the California Health and Safety Code (Sections 2274 et. seq.) the responsibility for the cost of mosquito control may fall on the property owner.